This invention relates to radio broadcasting and, more particularly, to methods of and apparatus for broadcasting digitally modulated signals and analog amplitude modulated signals within the same frequency channel assignment.
There has been increasing interest in the possibility of broadcasting digitally encoded audio signals to provide improved audio fidelity. Several approaches have been suggested, including out-of-band techniques in which the digital radio signals would be broadcast in a specially designated frequency band, and in-band techniques in which the radio frequency signals would be broadcast within vacant slots between adjacent channels in the existing broadcast band (interstitial approach) or within the same frequency channel allocations currently used by commercial broadcasters (in-band on-channel approach). The in-band approach may be implemented without the need for additional frequency coordination and with relatively minor changes to existing transmitting equipment. Of course, any digital audio broadcasting (DAB) technique should not degrade reception by conventional analog receiver circuits.
In-band approaches to digital audio broadcasting have thus far only been proposed in the FM band (88 MHz to 108 MHz), since the bandwidth of AM channels is quite narrow. However, the use of digital audio broadcasting in the AM band (530 kHz to 1700 kHz) would provide AM broadcasting stations with a means to compete with high quality portable audio sources such as cassette tapes and compact disc players. It would therefore be desirable to extend the in-band on-channel (IBOC) approach to AM broadcasting frequencies to provide enhanced fidelity through digital signalling without affecting reception by existing analog AM receivers.